This invention relates to the production of high density fuels by oligomerization of a mixture of cyclopentadiene dimer and methylcyclopentadiene dimer to their co-trimer followed by hydrogenation of the oligomerization product.
High density fuels are desirable to provide a high energy fuel source while at the same time minimizing the volume that the fuel occupies. Such fuels are important, for example, as fuels for certain military jet aircraft which have limited fuel storage capability. High density fuels permit maximizing the range of these aircraft by providing high energy per unit of volume occupied by the fuel.
To perform satisfactorily in such applications, the fuels must satisfy certain physical property requirements. The fuels must remain in liquid state at the low temperature experienced by aircraft flying at high altitude. This generally means that the fuel must have a melting point no greater than, if not substantially below -20.degree. C. To provide high energy per unit volume, it is generally desired that the fuels have a liquid density approaching or exceeding 1.0 gm/ml as compared to 0.75-0.8 gm/ml for kerosene. Liquid hydrocarbon fuels of this density necessarily contain multiple fused small rings, i.e., fused cyclic C.sub.3, C.sub.4 and C.sub.5 rings. However, most multiple ring compounds with a density of about 1.0 gm/ml have unacceptably high melting points. For example, the most common cyclopentadiene dimer has a melting point of 32.degree. C and its hydrogenated derivative has a melting point of 77.degree. C.
A few satisfactory compounds are known and have been utilized for these volume critical services. For example, U.S. Pat. No. 3,377,398 discloses a method for dimerizing bicyclo(2.2.1)hepta-2,5-diene over an iron complex catalyst. However, bicycloheptadiene must first be synthesized, generally by reaction between cyclopentadiene and acetylene. U.S. Pat. No. 3,381,046 discloses a process whereby a specific exo-stereo isomer of cyclopentadiene dimer and methylcyclopentadiene dimer is produced from and then isolated from the endo-isomers of cyclopentadiene dimer and methylcyclopentadiene dimer. The complexity of these processes makes the product high density fuel expensive and has limited the broader usage of these fuels.
U.S. Pat. No. 3,002,829 discloses a simpler process for making high density fuel by dimerizing mixtures of cyclopentadiene, methylcylopentadine and dimethyl cyclopentadiene, topping off uncoverted monomer, followed by a two step hydrogenation of the product. This process is comparatively simple, but tetrahydro dimer mixture produced does not possess as high an energy content as the products of the more difficult syntheses above.